1. Field of the Invention
This invention pertains to methods of conducting cementing operations in boreholes which penetrate subterranean earth formations. This invention still further pertains to a method of placing a balanced cement plug in a borehole.
2. Problems Solved
When drilling a borehole which penetrates one or more subterranean earth formations, it has, at times, been and continues to be, either advantageous or necessary to change the direction of the borehole as it is being drilled. In order to held in the change of direction, it has sometimes been a practice by those skilled in the art to place in the borehole, in the vicinity of the location therein where the change in drilling direction is to begin, a hardened mass of a hydraulic cement composition. This hardened mass of hydraulic cement is referred to in the art, and herein, as a sidetrack plug or as a kickoff plug or simply as a plug.
The specific function of a kickoff plug is to cause the drill bit to divert its direction. Accordingly, if the plug is harder than the adjacent formation, then the drill bit will tend to penetrate the formation rather than the plug and thereby produce a change in drilling direction. It should be noted that specific directional tools are also involved in drill direction changes, but such tools are not within the scope of this invention and will not be discussed.
A kickoff plug may fail to cause the drill bit to change direction if the plug is unreasonably contaminated with a foreign material, such as drilling mud, because drilling mud, when mixed in the set cement, can render the set mass softer than the adjacent formation. Accordingly, a problem encountered in all cementing operations is to remove foreign material, such as drilling mud, from the portion of the borehole to be cemented so as to avoid mixing drilling mud with the slurry of hydraulic cement and water. Thus, an object of this invention is to place in the desired location a competent plug of set cement. For purposes of this disclosure, a competent cement is a water slurry of hydraulic cement which does not promote solids separation, which is not contaminated with drilling fluid or any natural well fluid, such as formation water, gas or liquid hydrocarbon and which does not contain free water. A competent cement slurry, upon setting, will produce a homogenous mass of hardened cement.
Another object of this invention is to provide a method of introducing a slurry of hydraulic cement in water into the location of the borehole to be cemented in a manner which will avoid, or at least minimize, any mixing of the cement slurry with drilling mud present in the borehole while the drilling mud is being displaced therefrom by the cement slurry.
A kickoff plug may also fail to cause the drill bit to change direction, even if a homogeneous mass of cement is placed as desired, if the plug is not in sufficient contact with the wall of the borehole. Absence of sufficient contact between the set cement plug and the borehole wall can permit the plug to move (horizontally or vertically) or to rotate--and thus fail to resist the impact of the drill bit. Absence of sufficient wall contact, as mentioned above, can be caused by the existence of a layer of viscous drilling fluid or drilling solids, referred to herein as gelled mud or mud cake, respectively, between the sides of the plug and the wall of the borehole. It will be appreciated, that the existence of gelled mud or mud cake, as described above, has the effect of reducing the cross-section of the kickoff plug to a value less than the cross-section of the borehole. It is thus another object of this invention to provide a method of substantially removing gelled mud or mud cake from the walls of the borehole in the portion of the hole to be cemented so as to produce a plug having a cross section substantially the same as the cross section of the borehole.
For purposes of more specific definition, mud cake, as used herein, means a solid material, which adheres to the walls of a borehole, consisting of the solids content of a drilling fluid that has experienced the loss of all or of substantially all liquid initially used to carry or suspend the solids in the drilling fluid. It is thought, for example, that as a drilling fluid is pressed against the walls of a borehole the liquid is squeezed therefrom resulting in a cake of solids which adhere to the wall of the borehole.
Gelled mud, which is sometimes referred to as immobile mud, as used herein, means a drilling fluid that has developed, for various reasons, a gel strength, which is so high that the amount of energy required to move it is much greater than the amount of energy required to move drilling fluid which has a relatively much lower gel strength. For purposes of comparison, drilling fluid has a gel strength in the range of from about 0 to about 100 pounds per 100 square feet, whereas gelled mud has a gel strength greater than the drilling fluid being used.
As used herein, the terms drilling fluid and drilling mud have the same meaning and are used interchangeably. Gelled mud and mud cake, while being derived from drilling fluid, are not drilling fluid and have the meanings as above defined.
Known methods of producing kickoff plugs have traditionally produced plugs which have failed to successfully divert the drill bit to a new direction. It is not uncommon in the industry to produce two to four plugs before a successful diversion can be achieved. In some cases, as many as twelve kickoff plugs have been produced before a successful diversion has been achieved.
It is believed that the methods of forming the cement plug as disclosed herein will, if followed, produce a kickoff plug which will not fail to produce a successful drilling diversion and thus will eliminate the practice of forming multiple plugs in order to achieve a successful diversion of drilling direction. This invention thus solves the problems referred to above and satisfies a long felt, but, heretofore, unsolved need.